The performance of a helicopter can be limited by aerodynamic and/or dynamic considerations. The maximum gross weight at a given forward flight velocity, the maneuverability at a given forward flight velocity, and the maximum forward flight velocity can all be limited by retreating blade stall. Other aerodynamic considerations such as advancing blade drag divergence and pitching moment could also limit the maximum forward flight velocity.
The airfoil section requirements for a helicopter rotor are more complex than those for a fixed wing aircraft because, on a single revolution of the rotor, an airfoil section can experience lift coefficients from negative values to the maximum positive value and section Mach numbers from subsonic to transonic values. Since the ranges of lift coefficients and Mach numbers experienced by an airfoil section depend on its radial location along the rotor blade and the helicopter flight conditions, different airfoil sections have been used for a specified range of radial positions along the rotor blade.
The maximum lift coefficient of an airfoil section is of considerable importance in the process of selecting airfoils for application to a helicopter rotor. When the maximum lift coefficient of an airfoil section is exceeded (i.e., the airfoil is stalled), the corresponding drag coefficient increases dramatically and the pitching moment coefficient can change direction (nose-up to nose-down) as well as change greatly in magnitude. When a significant part of a rotor blade is operating beyond the maximum lift coefficient of the local airfoil section, the power required to sustain flight exceeds the power available, thus limiting the particular flight condition. As pointed out above, this could occur with increases in aircraft gross weight, in maneuvers, or in forward flight.
Prior art airfoil sections intended for use in the inboard region of a rotor blade or propeller are designed for high maximum lift coefficients and low pitching moment coefficients.
Accordingly, it is an object of this invention to provide an airfoil section which will operate at high lift coefficients before stalling (i.e., have high c.sub.1.sbsb.max) at Mach numbers from near zero to about 0.50 and which will simultaneously provide both near zero pitching moment coefficients about the quarter chord and low drag coefficients for a broad range of lift coefficients and Mach numbers.
It is a further object of this invention to provide near zero pitching moment coefficients about the quarter chord for lift coefficients from -0.2 to 1.0 for Mach numbers from near zero to 0.63 and to provide pitching moment coefficients less than 0.02 in magnitude for lift coefficients equal to or less than the maximum positive value (unstalled) for Mach numbers from near zero to 0.63.
It is a further object of the invention to provide a drag divergence Mach number in excess of 0.73 for lift coefficients from 0.0 to 0.30.
It is a further object of the invention to provide an airfoil section with a camber line that is used as the basis for a family of airfoils. That is, new airfoils of different maximum thicknesses are developed by a combination of the camber line of the airfoil sections described herein and a thickness distribution that is scaled from any one of those shown herein to provide specific maximum lift coefficients and drag divergence characteristics while retaining the near zero pitching moment coefficients.
It is a further object of the invention to provide improved rotor or propeller performance which results in fuel savings and/or increases in aircraft gross weight, maneuverability, and forward flight speed.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawing which follow.